Conventional data transmission using metal lines has limited transmission speed due to delay of a resistor-capacitor of a transmission line. In particular, the communication speed between a central processing unit (CPU) and a graphics processing unit (GPU) required for games has gradually increased. Thus, the demand for higher communication speeds has also increased. Silicon photonics has been suggested to satisfy this demand and can provide high integration. In particular, a monolithic device for optical communications can be manufactured to reliably transmit high-speed data generated inside a chip to an external chip. Also, an optical device using silicon photonics does not need to be additionally packaged, as opposed to using another device outside a chip. Thus, packaging cost can be reduced, and the operation speed is not limited by the packaging. As a result, high-quality data can be transmitted to an external chip.
However, elements such as a light source, an optical modulator, an optical light receiving device, a low noise amplifier, a limiting amplifier, an optical coupler, a multiplexer, a demultiplexer, an optical filter, etc. must be improved to use silicon photonics. The optical modulator is the core element.
An optical modulator has a maximum extinction ratio and a rapid switching structure. Also, the process of manufacturing the optical modulator is simple and may be compatible with a silicon process. For example, in a Mach-Zehnder structure obtaining an extinction ratio using an optical phase difference, an optical phase must be changed using a more efficient method to obtain a maximum extinction ratio.
An optical modulator having a metal-oxide-silicon (MOS) capacitor structure has been suggested for conventional silicon photonics.
FIG. 1 is a cross-sectional view showing the important parts of an optical modulator having a MOS capacitor type structure according to the prior art. Referring to FIG. 1, an optical modulator 10 having a MOS capacitor structure includes a silicon on insulator (SOI) substrate including a silicon substrate 12, a buried oxide layer (BOX) 14, and an n-type silicon substrate 16. A pair of n+-type doping areas 22 used as electron sources are formed on both sides of a p-type polysilicon layer 20, which is an optical beam path formed on the n-type silicon substrate 16. The n+-type doping areas 22 are grounded away from an optical condensing area 24 around the optical beam path.
Electrons and holes accumulate above and below a gate insulating layer 22 formed between the n-type silicon substrate 16 and the p-type polysilicon layer 20.
Metal contacts 32 are formed on dopant areas 34 connected to the p-type polysilicon layer 20 to apply a positive driving voltage VD to the dopant areas 34. An insulating layer 36 is formed between the metal contacts 32 and the optical condensing area 24.
In the optical modulator shown in FIG. 1, the optical condensing area 24 is formed mainly under the gate insulating layer 22, and a phase is delayed by the electrons accumulated under the gate insulating layer 22.
FIG. 2 shows an optical mode distribution of light condensed in the optical condensing area 24. Referring to FIG. 2, reference character A denotes a normalized optical profile obtained when the gate insulating layer 22 is relatively thin, and reference character B denotes a normalized optical profile obtained when the gate insulating layer 22 is thicker than in the normalized optical profile A.
As shown in FIG. 2, the maximum peak of the condensed light is positioned under the gate insulating layer 22.